support VURL backend

Not yet implemented is recording hashes on download from web and
verifying hashes.

addurl --verifiable option added with -V short option because I
expect a lot of people will want to use this.

It seems likely that --verifiable will become the default eventually,
and possibly rather soon. While old git-annex versions don't support
VURL, that doesn't prevent using them with keys that use VURL. Of
course, they won't verify the content on transfer, and fsck will warn
that it doesn't know about VURL. So there's not much problem with
starting to use VURL even when interoperating with old versions.

Sponsored-by: Joshua Antonishen on Patreon

Command/ImportFeed.hs

@@ -283,7 +283,7 @@ startDownload addunlockedmatcher opts cache cv todownload = case location todown
 	Enclosure url -> startdownloadenclosure url
 	MediaLink linkurl -> do
 		let mediaurl = setDownloader linkurl YoutubeDownloader
-		let mediakey = Backend.URL.fromUrl mediaurl Nothing
+		let mediakey = Backend.URL.fromUrl mediaurl Nothing (verifiableOption (downloadOptions opts))
 		-- Old versions of git-annex that used quvi might have
 		-- used the quviurl for this, so check if it's known
 		-- to avoid adding it a second time.
@@ -638,7 +638,7 @@ clearFeedProblem url =
 		=<< feedState url
 
 feedState :: URLString -> Annex RawFilePath
-feedState url = fromRepo $ gitAnnexFeedState $ fromUrl url Nothing
+feedState url = fromRepo $ gitAnnexFeedState $ fromUrl url Nothing False
 
 {- The feed library parses the feed to Text, and does not use the
  - filesystem encoding to do it, so when the locale is not unicode